head

[c70mel]

Hi guys,

Anyone have any ideas on how to do this?

Basically I am looking at using a relay to turn on the headlights or main beams. This relay will be activated by an Alarm/tracker unit which has 12v output triggers. I am thinking the main beam lights would be the best option to use as they can be turned on without the key being in (which is what we want).

Its a 1999 C70 2.4 coupe

Thanks in advance for any info/help!

[p fandango]

ah thats easy, all you need is 2 5-pole 40amp relays. Halfords or most car accessory shops have them for wiring fog/spotlights. You'll need a relay on each headlight

i haven't got a diagram so will explain it the best i can. On the relay there are 2 poles for activating the relay, & 3 on the switching (1 common pole, 1 connected when the relay is not tripped, 1 thats connected when the relay is tripped)

the alarm trigger needs splitting & connecting to 1 of the switching posts on each relay, the other switching pole needs connecting to either positive or earth depending what the alarm output is (if its positive then connect the pole to an earth, if its earth triggered the other pole will need to be a perminant positive)

remove the cap on the back of the headlight & find the wires to the headlight bulb you want off the alarm. Cut the positive wire & connect the bulb side of the cut wire to the common on the switching part of the relay. The loom part of the wire needs to go to the pole that is connected to the comon when the relay is non-active. Then the remaining pole which is connected to the common when the relay is active needs to be wired to a perminant live (with a inline fuse).

hope that makes sense, let me know if not & i'll try & explain it better

[c70mel]

That's great

Could you possibly get a product code for the relay from maplins? As I could pop in there on my way home today

[p fandango]

Could you possibly get a product code for the relay from maplins? As I could pop in there on my way home today
you want 2 relays AR32K LINK (http://www.maplin.co.uk/Module.aspx?moduleno=6436)

85 = wires to alarm trigger
86 = wires to either earth or perminant positive depending on alarm trigger
30 = wires to bulb side of cut wire
87 = wires to perminant live from battery
87a = wires to loom side of cut headlight wire

[c70mel]

excellent! got the last one they had in stock! so will have to just do one headlight for now.

Can you just confirm what pin 87a goes too? is that the wire that is 12v + when the main (fog) lights are on? Or is that pin 30?

Thanks again! great help!

[p fandango]

Can you just confirm what pin 87a goes too? is that the wire that is 12v + when the main (fog) lights are on? Or is that pin 30?
what you need to do is find the 2 original wires going to the light you want to use, one of the wires will be the earth & normally black ignore that wire you want the positive. With the positive wire cut it, now you've got 2 ends, the end which goes to the bulb goes onto pin 30, the other end of the cut wire which goes into the engine bay wiring loom now goes onto 87a

is that any clearer?

[c70mel]

Clear as ummm.. water now :)

Thanks very much for your help, as soon as it stops raining ill get my multimeter out and try and get it working!

Seeing as were on the subject of autoelectics do you know about diodes? I have read in the past (www.the12volt.com) that diodes are very commonly used. I have never really understood the main reason for them though to be used in car electics. I know the purpose of them it to only allow the current to flow in one direction but when would this be needed to be used in car alarm installations for example? I dont need to touch them for this little setup im doing with the headlights then?



what you need to do is find the 2 original wires going to the light you want to use, one of the wires will be the earth & normally black ignore that wire you want the positive. With the positive wire cut it, now you've got 2 ends, the end which goes to the bulb goes onto pin 30, the other end of the cut wire which goes into the engine bay wiring loom now goes onto 87a

is that any clearer?

[p fandango]

Clear as ummm.. water now :)

Thanks very much for your help, as soon as it stops raining ill get my multimeter out and try and get it working!
if you do have any problems or not quite understand it just let us now, you have PM :wink:


Seeing as were on the subject of autoelectics do you know about diodes? I have read in the past (www.the12volt.com) that diodes are very commonly used. I have never really understood the main reason for them though to be used in car electics. I know the purpose of them it to only allow the current to flow in one direction but when would this be needed to be used in car alarm installations for example? I dont need to touch them for this little setup im doing with the headlights then?
because of the high current of the headlights in this circuit you'd need to use relays anyway as the alarm trigger won't be powerfull enough, but if you just wanted to power the indicators you could of put the alarm trigger straight to the positive of the light with a diode to stop the alarm getting a back feed & powering the alarm when you put the indicator on. You'd also use diodes if you were linking 2 circuits together that are normally seperated, as in the indicator flash were you'd only got 1 trip, it would need to be split & a diode used to stop the left indicator flashing with the right as they are now connected through the alarm flasher circuit. Personally i've never used them & prefer to use relays, but most of my circuits are high current anyway & need relays

[c70mel]

if you do have any problems or not quite understand it just let us now, you have PM :wink:


because of the high current of the headlights in this circuit you'd need to use relays anyway as the alarm trigger won't be powerfull enough, but if you just wanted to power the indicators you could of put the alarm trigger straight to the positive of the light with a diode to stop the alarm getting a back feed & powering the alarm when you put the indicator on. You'd also use diodes if you were linking 2 circuits together that are normally seperated, as in the indicator flash were you'd only got 1 trip, it would need to be split & a diode used to stop the left indicator flashing with the right as they are now connected through the alarm flasher circuit. Personally i've never used them & prefer to use relays, but most of my circuits are high current anyway & need relays

Should i be using a diode to suppress any EMF from this relay? I dont think the output from the unit (alarm) is protected, I have attached a pic of some example schematics from the units manual

Thanks again

[p fandango]

Should i be using a diode to suppress any EMF from this relay? I dont think the output from the unit (alarm) is protected, I have attached a pic of some example schematics from the units manual
i've never used diodes in anything i've made, if the output you intend to use is designed as a trigger output then is should be insoluted from the other internal circuits